17β-hydroxysteroid dehydrogenases (17β-HSDs), also known as 17-ketosteroid reductases (17-KSR) are NAD(H)- and/or NAPD(H)-dependent alcohol oxidoreductase enzymes which catalyse the last and key step in formation of all estrogens and androgens. More specifically 17β-HSDs catalyse the dehydrogenation (oxidation) of 17-hydroxysteroids into corresponding 17-ketosteroids or hydrogenation (reduction) of inactive 17-ketosteroids into corresponding active 17-hydroxysteroids.
As both estrogens and androgens have the highest affinity for their receptors in the 17β-hydroxy form, the 17β-HSD/KSRs regulate the biological activity of the sex hormones. At present, 15 human members of 17β-HSDs have been described (type 1-15). Different types of 17β-HSD/KSRs differ in their substrate and cofactor specificities. The 17KSR activities convert low-activity precursors to more potent forms while 17β-HSD activities decrease the potency of estrogens and androgens and consequently may protect tissues from excessive hormone action.
Each type of 17β-HSD has a selective substrate affinity and a distinctive, although in some cases overlapping, tissue distribution.
Type 1 17β-hydroxysteroid dehydrogenase (17β-HSD1) is most abundantly expressed in the ovarian granulosa cells of the developing follicles in ovaries and in human placenta, both being estrogen biosynthetic tissues. In addition, 17β-HSD1 is expressed in estrogen target tissues, including breast, endometrium and bone. The human 17β-HSD1 is specific to estrogenic substrates and in vivo catalyzes the reduction of estrone to estradiol.
Type 2 17β-hydroxysteroid dehydrogenase (17β-HSD2) on the other hand converts estradiol, testosterone and 5a-dihydrotestrosterone to their less active forms estrone, androstenedione and 5a-androstanedione, respectively. Due to its wide and abundant expression in number of various estrogen and androgen target tissues, such as uterus, placenta, liver and the gastrointestinal and urinary tracts, it has been suggested that type 2 enzyme protects tissues from excessive steroid actions.
Estradiol (E2) is about 10 times as potent as estrone (E1) and about 80 times as potent as estratriol (E3) in its estrogenic effect. In contrast to certain other estrogens, estradiol binds well to both estrogen receptors ERα and ERβ, and thus regulates the expression of a variety of genes.
Although both 17β-HSD1 and 17β-HSD2 are present in healthy premenopausal humans, increased ratio of 17β-HSD1 to 17-HSD2 in the tumors of postmenopausal patients with hormone-dependent breast cancer has been shown in several studies. 17HSD1 gene amplification and loss of heterozygosity of 17HSD2 allele are potential mechanisms involved to increased reductive estrogen synthesis pathway in breast tumors. Increased ratio of type 1 enzyme to type 2 enzyme results in an increased level of estradiol that then promotes the proliferation of the cancerous tissue via the estrogen receptors (ER). High levels of estrogen thus support certain cancers such as breast cancer and cancer of the uterine lining i.e. endometrial cancer and uterine cancer.
Similarly it has been suggested that 17β-HSD2 is down-regulated in endometriosis while both aromatase and 17β-HSD1 are expressed or upregulated in comparison with normal endometrium. This again results in the presence of high concentration of estradiol (E2) which drives the proliferation of the tissue. Similar mechanism has been elucidated in uterine leiomyoma (uterine fibroids) and endometrial hyperplasia.
Reduction of the endogenous estradiol concentration in affected tissues will result in reduced or impaired proliferation of 17β-estradiol cells in said tissues and may thus be utilized in prevention and treatment of malign and benign estradiol dependent pathologies. Due to the proposed involvement of 17β-estradiol in a number of malign and benign pathologies, inhibitors of 17β-hydroxysteroid dehydrogenases, that can be used to impair endogenous production of estradiol from estrone, can have therapeutic value in the prevention or the treatment of such disorders or diseases are in great demand.
Some small-molecule inhibitors of 17β-HSD1 enzyme have been identified and reviewed in Poirier D. (2003) Curr Med Chem 10: 453-77 and Poirier D. (2010) Expert Opin. Ther. Patents 20(9): 1123-1145. Further, small molecule inhibitors of 17β-HSD's have been disclosed in WO 2001/42181, WO 2003/022835, WO 2003/033487, WO 2004/046111, WO 2004/060488, WO 2004/110459, WO 2005/032527, and WO 2005/084295.
WO2004/085457 discloses steroidal compounds capable of inhibiting 17β-hydroxysteroid dehydrogenase. WO2006/003012 discloses 2-substituted D-homo-estriene derivatives suitable for the treatment of estrogen-dependent diseases that can be influenced by the inhibition of the 17β-hydroxysteroid dehydrogenase type 1. Similarly WO2006/003013 presents 2-substituted estratrienones usable for preventing and treating estrogen-dependent diseases influenced by inhibiting 17β-hydroxysteroid dehydrogenase type 1.
15-substituted estradiol analogues acting as locally active estrogens are presented in WO2004/085345. WO2006/027347 discloses 15b-substituted estradiol derivatives having selective estrogenic activity for the treatment or prevention of estrogen receptor-related diseases and physiological conditions. Further, WO2005/047303 discloses 3, 15 substituted estrone derivatives capable of inhibiting the 17β-hydroxysteroid dehydrogenase type 1.
International application WO2008/034796 relates to estratrien triazoles suitable for use in treatment and prevention of steroid hormone dependent diseases or disorders requiring the inhibition of a 17β-hydroxysteroid dehydrogenases such as 17β-HSD type 1, type 2 or type 3 enzyme. Inhibitors of 17β-HSD type 3 enzyme have been disclosed in WO99/46279.